Prescribed fire alters nematode communities in an old‐field grassland

Abstract Fire is a common disturbance in many biomes, with both beneficial and detrimental effects on soil biology, which largely depend on fire intensity. However, little is known about the impact of fire on soil nematode communities in terrestrial ecosystem. In the present study, we investigated the effects of short‐term prescribed fire on soil nematode communities and soil properties in an old‐field grassland in Northern China. The results showed that burning significantly increased soil nematode abundance by 77% and genus richness by 49% compared to the control. Burning also decreased taxon dominance by 45% (Simpson's D) and increased nematode diversity by 31% (Shannon‐Weaver H'). However, burning increased plant parasites (particularly genera Cephalenchus and Pratylenchus) and shifted community to more bacterial‐feeding genera (i.e., decreased Channel Index). Generally, burning increased soil bio‐available nitrogen (NH4 +–N and NO3 −–N) content, which would be the main drivers causing nematode community to flourish via a “bottom‐up” effect. These results suggest that prescribed fire increases nematode diversity and alters community composition toward more plant parasites and bacterial feeders. Our findings highlight the importance of prescribed fire management in shaping short‐term nematode community structure and function, but the long‐term effects and impacts of these changes on soil nutrient and carbon cycling remain unknown.


| INTRODUC TI ON
Fires regulate the structure and function of terrestrial ecosystems, due to their profound effects on vegetation structure and composition, nutrient cycling, and herbivore distribution (Augustine et al., 2014;Bond & Keeley, 2005;Pellegrini et al., 2018;Thoresen et al., 2021). Indeed, some ecosystems are even fire-dependent and consequently require fire to ensure their successional development (Barnes et al., 2022). At the same time, fire can be used as a landscape management practice, including the annual or semiannual burning of grasslands, clearing of debris and encouraging fire-dependent tree seedling growth, and also used for residue removal in agriculture (Gomes et al., 2020;Thomaz et al., 2014). Given the importance of fire for managing ecosystems and the increasing frequency and intensity of unintentional fires with climate change (Pellegrini et al., 2018), it is critical to understand the consequences of fire for below-ground ecosystem structure and function.
Most studies that consider the effects of fire on belowground ecosystems mainly focused on soil biogeochemistry and microbes (e.g., bacteria, fungi) generally (Pressler et al., 2019). Soil nematode, however, are critical to soil and general ecosystem function as they prey upon these soil microbes and thereby cycling soil nutrients (Osler & Sommerkorn, 2007;Semeraro et al., 2022). Soil nematode could prevent or cause plant diseases (Wu et al., 2021) and are key predators preying other soil micro-fauna (Kostenko et al., 2015;Peguero et al., 2019). Neglecting prescribed fire effects on soil nematodes prevents a full picture of the belowground effects of fire on ecosystem structure and functioning (Gomez et al., 2022;Zaitsev et al., 2016).
Nematoda are a widespread and dominant phylum of soil invertebrates, accounting for more than 80% of the multicellular animals in the world (Bloemers et al., 1997). Nematodes colonize nearly all of earth and include a large number of genera with a wide range in size and morphology. They are most commonly known and characterized by their stylet-the specialized organ used for feeding.
Nematode communities include almost all trophic groups except for the producer group, and they may feed on bacteria, fungi, or parasitize host plants or animals, according to their feeding habit (Butenko et al., 2017;Moll et al., 2021). Nematodes are also highly sensitive to environmental changes. Nematode community richness and composition respond rapidly to chemical and physical changes (Nielsen et al., 2014). Therefore, nematodes are frequently used as bioindicators of the quality, stability, and changes in the soil (Devi, 2020;du Preez et al., 2018). Therefore, understanding the soil nematodes response to fire is helpful to realize how ecosystem processes are likely to respond to fire.
Previous studies showed inconsistent effects of fire on soil nematode communities. For example, a 4-year study revealed that fire had strong negative effects on soil nematode community and even reduced soil nematode abundance by 76% in a semiarid grassland (Bastow, 2020). While there are also studies on forest soils affected by fires found that fire significantly increased bacterial-feeding nematodes abundance even by 640% higher than the unburnt sites (Pen-Mouratov et al., 2012). Reasons for such inconsistencies are still unclearly, but may be due to environmental (e.g., temperature, pH, N availability) or management differences (e.g., fire intensity) (Matlack, 2001;Trouvé et al., 2020;Zaitsev et al., 2016). In general, this observation is somewhat counterintuitive since many studies demonstrated that soil microbial communities after fires shift from fungi-dominated to bacteria-dominated ones (Dooley & Treseder, 2012;Mikita-Barbato et al., 2015). One can assume that such controversies may be driven by overarching regional differences in their food resource availability and quality, but it is still not known well (Butenko et al., 2017).
In recent decades, major socioeconomic changes in China have led to rural exodus and land abandonment. Agricultural land has been taken out of production and natural regeneration has occurred. As a result, this has produced a series of old-field successional communities in northern China (Cramer et al., 2008). An inevitable consequence of this successional development has been increased vegetation biomass and hence an added fire risk. Most previous studies of prescribed fire effects on old-field ecosystems mainly focus on responses of soil physicochemical properties and aboveground ecological processes (Dowhower et al., 2021). How soil nematodes respond to fires in old-field grassland ecosystems is largely unknown.
To better understand the effects of prescribed fire on soil nematode communities, we conducted an experiment comparing burned and control treatments in an old-field grassland in Northern China to address the following questions: (1) (Song et al., 2017). Because these sandy-loam soils are conducive to greater evaporation combined with the shallow groundwater, slow surface runoff, and high salinity-they are strongly alkaline (pH = 8.7) even though the rainfall is plentiful in this region (Song et al., 2016).

| Experimental design and sampling
Ten 3 m × 3 m plots were established in the present study. Each plot had 2 m wide buffer zones with removed vegetation and litter to prevent fire spread. Five plots were randomly selected for prescribed burning. For the burning treatment, we fired the withered grass with a blowtorch and burned itself out to mimic the nongrowing season natural fire on December 3, 2018. Soil samples were collected on March 8, 2019. Four cores were randomly taken in each plot with a soil auger (5 cm diameter, 10 cm in depth) and mixed carefully to obtain a total of 10 composite samples. Stones, large roots, and macroarthropods were excluded by hand. Each soil sample was divided into three subsamples. The first subsample was used to analyze soil physicochemical properties including soil gravimetric water content, SOC, soil total N (TN), soil pH, NO 3 − -N, and NH 4 + -N. The second subsample was used to measure microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) concentration. The third subsample was used to extract and identify soil nematodes.

| Sample analyses
A total carbon analyzer (Vario MACRO CUBE, Elementar Inc.) was used to analyze soil organic C and total N contents. Soil organic C was determined by the Walkley-Black's wet digestion method with a Total Organic Carbon Analyzer (Elementar Vario TOC, Elementar Co.). The pH values were determined with a combination glass electrode (soil/ water W/V ratio 1:2.5). Microbial biomass was measured by fumigationextraction method (Vance et al., 1987). Soil plant-available, inorganic
Nematode data were also used to calculate Maturity Index (MI) (Yang et al., 2018), Channel Index (CI), Enrichment Index (EI), and Structure Index (SI) (Ferris et al., 2001). Lower and higher values of MI indicate disturbed and stable nematode communities, respectively. CI was calculated to evaluate the relative functional intensity of soil decomposition pathways. A low CI indicates the dominance of the bacterial channel, while a high CI refers the dominance of the fungal channel. A high EI suggests a resource-enriched soil ecosystem. A high SI indicates a complex and stable food web.
The diversity and ecological indices were calculated as following. 6. Structure index: SI = 100 × ∑ k s n s /(∑ k b n b + ∑ k s n s ) (Ferris et al., 2001), where n b is the abundance of individuals and k b is the weighting in guilds Ba 1 and Fu 2 , which represent the basal characteristics of soil food web; k s is similar weighting assigned to Ba 3 -Ba 5 , Fu 3 -Fu 5 , Op 3 -Op 5 ; and n s is the abundance of above- values and feeding groups following Bongers and Bongers (1998).

| Statistical analysis
The nematode population density and genus richness were We used principal components analysis (PCA) to determine the prescribed burn effect on the overall nematode community in R (version 3.4.3). Using prcomp in vegan package (Dixon, 2003;Oksanen et al., 2007), we analyzed for community similarity/dissimilarity. We then used the envfit function to correlate ancillary properties (e.g., plant biomass and ancillary soil properties) with nematode community. Finally, a multiple ANOVA, or MANOVA, was conducted using manova function with top three principal components from the PCA. Treatment differences at the level of p < .05 were considered as significant, but correlates in PCA were mapped with less stringent value of p < .1.

| Nematode abundance and diversity
Across both treatments, we found 29 taxa from 21 nominal families with frequencies of detection ranging from <99% to >1% of the samples (Table S1). Their feeding habits included bacterial feeding (Ba), fungal feeding (Fu), plant parasites (PP), and omnivorespredators (Om). Nematode taxa at the experimental site represented all the cp (structural guild) categories (Bongers & Bongers, 1998).
The prescribed burn had strong effects on overall abundance and measures of diversity ( Figure 1 and Table 2). Burning increased abundance from 258 ± 43 to 458 ± 88 (mean ± standard error) individual nematodes per 100 g soil, or a 77% increase from the control.

Most notable from diversity indices, burning decreased Simpson
Dominance Index (D) and increased Shannon-Weaver Index (H′) by 45% and 31%, respectively.

| Nematode trophic groups and ecological indices
Prescribed burning had striking effects mostly on each trophic group. Burning significantly increased relative abundance of PP by 98% compared to the control (Figure 2).
Prescribed burning had mixed effects on ecological indices (Table 3). Burning increased the EI and the MI, decreased the CI, while had no effect on the SI (

| Genera composition of nematode community
Prescribed burning had notable effects on nematode genera ( Figure 3) Figure S1). Plotting PC1 vs. PC2 explained 45.7% of variation and showed distinct separation between control and burned nematode communities. A MANOVA of top three PCs showed that community composition was significantly different (p = .036). Soil MBC, nitrate, and ammonium were positively correlated with prescribed burn soil nematode community, but pH negatively correlated (p < .1, Figure 4). This can also be confirmed with univariate correlations between these soil properties and both abundance and genus richness ( Figure S2).

| The underlying effect of fire intensity on soil nematode abundance and diversity
To our surprise, contrary to our hypothesis and prior studies, we found prescribed burning increased soil nematode abundance and richness (Figure 1). Most studies show that fire can decrease soil nematode abundance and richness even by 88% and 96%, respectively (Bastow, 2020;Pressler et al., 2019;Whitford et al., 2014). However, other studies have shown significant positive (+35%) or neutral effects of fire on nematodes (Čerevková et al., 2013;Matlack, 2001).

F I G U R E 3
Relative abundance of dominant nematode genera. Dominant genera were deemed those where at least one treatment showed >1% of total abundance. Those with both treatments <1% were grouped into 'Miscellaneous' for ease of visualization. Trophic group categories are shown to the right. Abundances are means (n = 5).
F I G U R E 4 Principal component analysis (PCA) of abundance of nematode genera (individuals per 100 g). Principal components (PC) 1 and 2 shown on LEFT, and PC 2 and 3 shown on RIGHT. The percentage of variance explained for each PC shown in parentheses. Red arrows with labels show correlated ancillary soil properties and environmental variables at (p < .1).
These conflicting responses of soil nematodes to burning probably related to either environmental factors (soils and/or climate) or the intensity of the fire. With fire intensity being the most parsimonious explanation (Whitford et al., 2014). Previous studies have shown that the responses, positive or negative, of soil organic matter (SOM), and microorganism to fire are related to the intensity (Alcañiz et al., 2018).
More specifically, low intensity fire can increase soil resource availability and microbial biomass, thus effectively stimulating the activity of soil nematode (Liu et al., 2007;Úbeda et al., 2005). On the contrary, high intensity fire will reduce soil resource availability, and then restrain the supply of nematode nutrition resulting in low soil nematode abundance and richness (Hinojosa et al., 2021;Shakesby et al., 2015).
We did not address the fire intensity in our study, but from this we might expect we had lower-intensity fire with lower maximum temperatures since both nematode abundance and microbial biomass increased with burning ( Figure 4).

| The main factors driving soil nematode community changes
While fire can have direct effects on soil biota with increased temperatures, there are also indirect effects of fire that many have documented. In our study, prescribed burning altered some environmental variables that have known effects on soil nematode communities. While these relationships do not infer causation, they can help lead hypothesis generation for why nematode communities change with prescribed fires.
The effects of these increased levels of highly available N during and following fire are often beneficial to the recovering plants and microbial populations by providing more bio-available nutrients (Fultz et al., 2016;Gray & Dighton, 2009;Liu et al., 2007). In our research, the increasing MBN is direct proof for that. As a result, these abundant N-rich food sources would cause the nematode community to flourish via a "bottom-up" effect.
The changing soil microenvironment is another important reason for the increase of soil nematode abundance. Soil pH generally increases following fires (Certini, 2005), but decreases have been observed in lower intensity fires (Pereira et al., 2017). Burning decreased soil pH in our study by 1.5% (Table 1). In coarse-textured sandy soils, the cations released from organic matter during the fire are easily lost from soil due to the surface runoff and erosion, resulting in a decrease in soil pH. In addition, increases in microbial activity (SOM decomposing) and nitrification can also lower pH via proton generation from these biological processes. In order to regulate their osmotic pressure, nematodes exchange several ions through their cuticle (Castro & Thomason, 1971). Changes in soil pH, due to burning, could have led to indirect effects on the nematode community (Korthals et al., 1996;Liang et al., 2020;Räty & Huhta, 2003).
In our research, the increase of soil temperature during the burn and the increase of radiation resulting from dark-colored fuel residues are also important reasons for the increase of soil nematode (Convey & Wynn-Williams, 2002). In addition, the decreasing soil pH move soil pH closer to neutral would benefit most nematode genera (Burns, 1971;Oka et al., 2000), which means soil pH have bigger indirect impacts on nematodes.

| The effect of fire on nematode functional and trophic groups
Prescribed fire significantly altered the nematode community structure (Figures 2 and 3). In particular, burning increased the proportion of PPs by 98% compared to the control ( Figure 2). This observation was consistent with previous study showing prescribed fire increased plant pathogen nematodes by 20% in a tall tussock grassland ecosystem after 16 months since last burnt (Yeates & Lee, 1997).
The PP genus with greatest increase was Cephalenchus, a widespread genus, its proportion in the burned area was twice as much as that in the control area. In general, plants inhabiting burnt areas are faster growing due to fire-induced availability of resources, such as light, space, and nutrients (Butenko et al., 2017). In contrast to PP, burning decreased the relative abundance of Fu (and Ba), albeit insignificantly ( Figure 2). This could be attributed to the fungi, in particular, are more sensitive to heat and slower to return to pre-fire levels (Pressler et al., 2019). Previous studies have shown that heating soils can release chemicals that inhibit fungal growth (Choromanska & DeLuca, 2001;Díaz-Raviña et al., 1996).
Burning increased the Shannon-Weaver index (H') but suppressed the dominance (Figure 1). This was mainly due to the fact that burning increased genus richness but reduced the proportion of dominant genera (e.g., Acrobeloide and Aphelenchoides). In terms of ecological indices, burning positively affected the EI and the CI, but had no effects on the SI and MI ( Table 3). The values of EI could reflect the increased availability of resources to the soil food web and the response of primary decomposers to the resources, which has been found to directly relate to the cumulative net N mineralization (Ferris & Matute, 2003). In this study, burning decreased the CI by 75.7% (Table 2). The changes in CI could be largely explained by the more dramatic decrease in the relative abundance of fungivores than bacterivores. This finding is consistent with meta-analysis comparison that reported that bacterial community changes were more resistant to fire than fungi across the literature (Pressler et al., 2019), and thus CI has been used as an indicator of bacterial-vs-fungal decomposition pathways (Ferris et al., 2001). Low CI values indicate bacterial dominated decomposition whereas high values refer to a more fungal dominated system. In this experiment, the lower CI implies that burning drives the soil food web to bacterial decomposition channels.

| CON CLUS IONS
Fire can have destructive effects on soil nematode communities, as most comprehensively shown in meta-analysis. However, we found controlled burning had positive effects on nematode abundance and diversity after 3 months. This unique finding could be due to edaphoclimatic context of our study, management factors like intensity of our prescribed burn, or both factors.
While controlled burning increased nematode abundance and diversity, it also increased the relative abundance of plant pathogens. In particular, it increased the abundance of Cephalenchus.
The long-term impacts of this disturbance and change in nematode communities are unknown, but it may have short-term (1-2 years) or even long-lasting impacts like increasing parasitation of the recovering plant community. Our findings highlight the importance of prescribed fire management in shaping short-term nematode community structure and function. However, the long-term effects remain unclear and warrant further exploration, in particular how they might be related to these short-term changes in nematode community dynamics.

ACK N OWLED G M ENTS
This study was supported by the National Natural Science Foundation of China (41701283), China Postdoctoral Science Foundation (2019T120621).

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare that they have neither personal relationships nor competing financial interests that could affect the work.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available in Dryad at 10.5061/dryad.j3tx95xk9.